Sailor Iron Mouse is the first of the Sailor Animamates to appear. Her human pseudonym in the anime is Chuuko Nezu, which is a pun. In Japanese, Chuuko and Nezu are each perfectly acceptable names; however, "chuu" is the sound made by mice, and nezu can also mean "mouse." True to her name of mouse, Iron Mouse is very short, at least a foot shorter than her targets (Takeuchi's notes in the manga state that she is Chibiusa's height, but she is not quite that short in the anime). She is also afraid of cats.

Iron Mouse has a very short life in the manga, only a few pages long. She appears in Act 43, in the Fantastic Music Hall during the joint concert between the Three Lights and Michiru Kaioh. She attacks with "Galactica Crunch" and is killed by Sailor Star Healer. It is later revealed that Sailor Iron Mouse killed the Senshi of her home planet, Sailor Chuu.

In the anime, Iron Mouse's personality is both childlike and intense; she usually chooses famous or impressive people as her victims, seems to enjoy her work, and is openly terrified of Galaxia. She can warp from place to place using a phone booth that she summons and she can also use it to block attacks. Sailor Iron Mouse is the antagonist for several episodes of the anime, but in episode 181 is killed by Galaxia as punishment for her failure to collect starseeds, even though she had discovered the identity of Sailor Star Fighter.